The Synchronous Digital Hierarchy (SDH) and its North-American equivalent, the Synchronous Optical Network (SONET), are the globally accepted, closely related and compatible standards for data transmission in the public wide area network (WAN) domain. Recently, SDH/SONET has also been adopted by the ATM Forum as a recommended physical-layer transmission technology for ATM (Asynchronous Transfer Mode) network interfaces.
SONET and SDH govern interface parameters; rates, formats and multiplexing methods; operations, administration, maintenance and provisioning for high-speed signal transmission. SONET is primarily a set of North American standards with a fundamental transport rate beginning at approximately 52 Mb/s (i.e., 51.84 Mb/s), while SDH, principally used in Europe and Asia, defines a basic rate near 155 Mb/s (to be precise, 51.84×3=155.52 Mb/s). From a transmission perspective, together they provide an international basis for supporting both existing and new services in the developed and developing countries.
For transmitting data, SDH and SONET use frame formats transmitted every 125 μs (8000 frames/s). Because of compatibility between SDH and SONET, their basic frames are similarly structured, but differ in dimension, which fact reflects the basic transmission rates of 155.52 and 51.84 Mb/s, respectively. To be more specific, a basic frame format of SDH is 9 rows of 270 bytes, or 2430 bits/frame, corresponding to an aggregate frame rate of 155.52 Mb/s. For SDH systems, the mentioned basic frame transmitted at the rate 155.52 Mb/s forms the fundamental building block called Synchronous Transport Module Level-1 (STM-1 which, according to SDH mapping scheme, contains a signal called AU-4 which, in turn, carries a signal VC-4). For SONET systems, the basic frame has dimensions of 9 rows by 90 bytes (270.3) and, being transmitted at the rate 51.84 Mb/s (155.52:3), forms the appropriate fundamental building block called Synchronous Transport Signal Level-1 (STS-1 containing AU-3 that carries a signal VC-3).
Each basic frame of SONET or SDH comprises an information portion called Information Payload and a service portion called Overhead (OH). Information payload is usually formed by virtual container signals VC4, VC3 and the like, and comprise a so-called POH (Path Overhead) portion predestined for various service and control functions.
SDH comprises also signals of Synchronous Transport Level 4, 16 and 64, which constitute 4, 16 or 64 independent VC4 signals. An analogous arrangement exists in SONET (signals STS1, STS3, STS12, STS48 etc.)
SDH and SONET are known to support data streams having rates higher than the fundamental building block. If there are services requiring a capacity greater than 155 Mbps, one needs a vehicle to transport the payloads of these services. In SDH, so-called concatenated signals, for example VC4-Nc, are designed for this purpose. STM-4 signal having a data rate 622.08 Mb/s (4×155.52 Mb/s) is one of the high order signals in the SDH system. Payload of the STM-4 signal is generated by byte-interleavingly multiplexing four payloads of STM-1(or four AU4, or four VC4) signals. Concatenated VC4 (VC4-Nc) is characterized by a common synchronous payload envelope being N-fold VC4-s, and by a common POH. Concatenated signals can be transmitted over a network in a number of ways.
So-called contiguous concatenation means that the high rate signal is transmitted as a continuous stream over a single path; in that case the signal's combined payload is sent in one synchronous payload envelope (SPE) having a common POH column in the standard frame.
Another option of transmitting a concatenated signal is dividing it into a number of fragments and transmitting them in parallel via respective channels, for further assembling at the destination point. Such a technology is generally known as a so-called Inverse Multiplexing (Inverse-MUX technology), which is applied when a suitable data channel is not available for a high rate data signal. Inverse Multiplexing is in use, for example, for PDH and ATM signal transmitting. In one particular implementation, one PDH signal of 10 MHz can be divided into five “fragment” signals of 2 MHz transmitted via respective five parallel paths; likewise, it may be divided into 3 individually transmitted signals of 2 MHz and 4 parallel fragments of 1 MHz each.
In SDH/SONET, the high rate contiguous concatenated stream is transformed into a so-called virtual concatenated signal by disassembling the contiguous signal and transmitting its lower rates fragments along respective parallel paths. For example, for transmitting VC4-4c, it is disassembled into four VC4 signals, which are transmitted via four individual paths, wherein the VC4 signal is considered a fragment. At the destination point, the fragments must be re-assembled to form the initial contiguous concatenated signal. This option is proposed in an ITU-T Standard Recommendation G.707 for transmission of concatenated signals over telecommunication networks. Of course, the fragments must be frame-aligned prior to re-assembling and the efficiency of transmission wholly depends on the proper synchronizing.
Therefore, a problem arises how to synchronize the transmission of fragments of the disassembled concatenated stream via different paths that usually have different delays. Moreover, the delays may change. Reasons for the delay fluctuations may be a newly introduced or removed network element, etc.
Indeed, when the fragments arrive to the point of re-assembling, the delay difference there-between may become so great and/or disordered that it will prevent the proper restoration of the initial stream.
A method is presently discussed in various Standard Committees of how to handle the above problem at the point of re-assembling. (For example, a contribution ETSI TM1/WP3/SDH of Nortel Networks submitted for discussion at Sophia Antipolis on 23–26 Nov. 1998).
The solutions presently discussed are based on providing a number of buffer memory blocks situated at equipment of the destination point (say, four blocks in the case of VC4-4c transmitted via four parallel. paths). In the case of VC4-4c, each of the four memory blocks is assigned to a respective fragment of the concatenated stream; let VC4 fragments of the concatenated data stream are designated as A, B, C and D streams. When these VC4 fragments, initially belonging to one concatenated VC4-4c signal, arrive to the respective buffer memory blocks at the destination point, they usually demonstrate a delay difference which did not exist at the time of their simultaneous “launching”.
It should be noted that data frames in any fragment (A, B, C or D) are arranged in multiframes, each multiframe comprising a particular number of frames. Each frame in the multiframe has its serial number indicated in the overhead portion of the frame (POH of VC4). The device at the destination point, receiving the individually coming fragments (VC4 streams) online, will be capable of proper aligning and reassembling them into the initial synchronous envelope only when their inter-relationship can still be correctly restored. It is good if the delay conditions of different parallel paths are more or less the same, and the fragments' streams arrive to the device quite “close” to one another from the point of their frame number in the multiframe. For example, the beginning of frame No. 1 in the first multiframe of fragment A arrives with a particular delay relative to the beginning of frame No. 1 in the first multiframe of fragment B, while the beginning of frames No. 1 in the first multiframes in fragments C and D arrive simultaneously but much earlier than those of stream B. The assumption is made that the maximal delay difference does not exceed a half of one multiframe, thus the buffer blocks of fragments A,B,C and D at the destination point deal simultaneously with frames belonging to multiframes which were sincronously launched at the source point. To support considerable delay differences, longer multiframes may be formed. The longer the expected delay, the greater the buffer which should be installed at the destination point.
The presently known manner of handling the realignment is simple: the VC4 fragments of the above example, which arrived at the destination point earlier (i.e., their frame No. 1 in a multiframe is detected earlier) are held in the buffer memory blocks of the respective paths until other VC4 fragments' “parallel” frames arrive to the realignment device along their respective paths. Reassembling is performed at the output of the different buffers.
However, if the delay difference between the VC4 streams, following along the parallel paths approaches and even exceeds a ½ cycle period (being ½ the length of the multiframe), the delay difference between the VC4 streams will become indistinguishable and the correct order will be lost.
In other words, with a given equipment at the destination point (the point of re-assembling fragments of the concatenated stream), the problem may become unsolvable at a particular moment when one or more of the parallel channels change their delay.